1. Field of the Invention
The present invention relates to an optical information recording/reproduction apparatus for recording information on an information recording medium such as a magnetooptical information recording medium or/and for reproducing information from the information recording medium and, more particularly, to a detection mechanism for detecting a focusing error signal (to be referred to as an AF error signal hereinafter) and a tracking error signal (to be referred to as an AT error signal hereinafter), or/and a magnetooptical detection signal (to be referred to as an MO signal hereinafter) in an optical system of the apparatus.
2. Related Background Art
Conventionally, various methods have been proposed for so-called AF error signal detection for guiding a detection light beam reflected by an information recording medium such as a magnetooptical information recording medium onto the light-receiving surface of a photodetector, and detecting the focusing state of a light spot formed for recording/reproduction on the information recording medium. Of these methods, a beam size method for detecting a change in beam spot size of the detection light beam is an excellent method since it requires only a small number of components in an optical system and an electrical control system.
For example, FIG. 1 shows an example (the invention disclosed in Japanese Laid-Open Patent Application No. 60-256929) of an AF error signal detection optical system based on the conventional beam size method. Referring to FIG. 1, a light beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and the collimated beam reaches an objective lens 4 via a beam splitter 3. The collimated beam is converged by the objective lens 4 to form a light spot on the recording surface of an information recording medium 5. Light (detection light beam) reflected by the information recording medium 5 is transmitted through the objective lens 4 again, is deflected by the beam splitter 3, and is split into a reflected light beam and a transmission light beam by a beam splitter 7-a. The reflected light beam is converged by a convergent lens 6-a. The transmission light beam is deflected by a mirror 7-c in substantially the same direction as the reflected light beam, and is converged by a convergent lens 6-b. Thereafter, the split light beams are respectively incident on photodetectors 10-a and 10-b.
The light-receiving surfaces of the photodetectors 10-a and 10-b are arranged, so that one photodetector receives the corresponding light beam at a position shifted from a focal plane 25 of a corresponding one of the convergent lenses 6-a and 6-b in a direction to approach the convergent lens, and the other photodetector receives the corresponding light beam at a position shifted from the focal plane 25 in a direction to be separated away from the corresponding convergent lens. Therefore, on the light-receiving surfaces of the photodetectors 10-a and 10-b, the beam spot sizes change in opposite directions in accordance with a change in focusing state of the light spot (recording/reproduction light spot) on the information recording medium. More specifically, when the objective lens 4 is defocused toward the information recording medium 5, if the beam spot size on the photodetector 10-a increases, the beam spot size on the photodetector 10-b decreases. When the objective lens 4 is defocused in the direction to be separated away from the information recording medium 5, the relationship between the beam spot sizes on the photodetectors 10-a and 10-b is reversed to that described above. Thus, a difference between signals output from the photodetectors 10-a and 10-b is calculated to obtain an AF error signal.
To summarize, since light beams incident on the photodetectors 10-a and 10-b can be split by the polarization beam splitter 7-a in units of components in a direction of 45.degree. with respect to the original plane of polarization, differential detection of an MO signal can also be simultaneously achieved by differentially calculating these components.
FIG. 2 shows another example (based on a method disclosed in, e.g., Japanese Laid-Open Patent Application No. 61-198456 or 61-198457) of an AF error signal detection optical system. Referring to FIG. 2, a light beam emitted from a semiconductor laser 1 as a light source is collimated by a collimator lens 2, and is deflected by a beam splitter 3 toward an objective lens 4. The deflected light beam is converged by the objective lens 4 onto the recording surface of an information recording medium 5. Light (detection light beam) reflected by the information recording medium 5 is transmitted through the objective lens 4 and the beam splitter 3 again, and then reaches a beam splitter 7-a via a halfwave plate 20. The light beam is then split by the beam splitter 7-a into a reflected light beam and a transmission light beam. Of the two split light beams, the transmission light beam is reflected and deflected by a reflection surface 7-b, so that the two split light beams emerge in the same direction. The two light beams are transmitted through the pupil range of a single convergent lens 6 from different positions, and become convergent light beams. The two convergent light beams are incident on a single photodetector 10 for detecting an AF error signal. In this case, since the photodetector 10 is arranged at a position shifted from the focal plane of the convergent lens, the light beams reach the photodetector 10 while being spatially separated from each other.
FIG. 3 is a diagram for explaining two photodetection regions of the photodetector 10 and detection signal processing in the prior art shown in FIG. 2. The two light beams are illustrated as hatched circles on the corresponding photodetection regions. Each photodetection region is divided into three portions (light-receiving portions U, V, and W or X, Y, and Z). If signal components from these light-receiving portions are represented by IU, IV, IW, IX, IY, and IZ, as is well known, an AF error signal can be calculated by the following equation upon execution of arithmetic processing via pre-amplifiers 13-a to 13-c, summing amplifiers 14-a and 14-b, and differential amplifiers 15-a and 15-b: EQU AFE=(IV-(IU+IW))+(IY-(IX+IZ))
In order to detect the tracking state of a light spot with respect to an information track on an information recording medium by guiding a detection light beam reflected by the information recording medium onto the light-receiving surface of a photodetector, a method of detecting an AT error signal is used. This method normally adopts a known push-pull method, and conventionally, various arrangements for detecting a push-pull signal have been proposed.
FIG. 4 shows an example (e.g., the invention disclosed in Japanese Laid-Open Patent Application No. 62-132242) of an AT error signal detection optical system based on the conventional push-pull method. Note that the example shown in FIG. 4 includes an AF error signal detection mechanism. Referring to FIG. 4, a light beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and the collimated beam reaches an objective lens 4 via a beam splitter 3. The collimated beam is converged by the objective lens 4 onto the recording surface of an information recording medium 5. Light (detection light beam) reflected by the information recording medium 5 is transmitted through the objective lens 4 and the beam splitter 3 again, and only light components associated with a beam spot size in a direction parallel to the track direction (e.g., the circumferential direction if the information recording medium 5 has a disk shape) of the recording medium are converged by a cylindrical lens 26. Then, a light beam emerging from the cylindrical lens 26 is split into two beams by a beam splitter 30, and the two beams respectively reach phOtodetectors 10-a and 10-b. The light-receiving surfaces of the photodetectors 10-a and 10-b are arranged at positions separated by a distance W from a plane 25, where the caustic curve of the cylindrical lens 26 is formed, in directions along the optical axes of the incident light beams. More specifically, the light-receiving surface of the photodetector 10-a is shifted in a direction in which the light beam approaches the beam splitter 30, and the light-receiving surface of the photodetector 10-b is shifted in a direction in which the light beam is separated away from the beam splitter 30. FIG. 4(a) shows how to split the photodetection regions of the photodetectors 10-a and 10-b by attaching portions shown by reference numerals 31-a and 31-b to the sides of the photodetectors 10-a and 10-b. The split light-receiving portions 31-a and 31-b of the photodetection regions of the photodetectors 10-a and 10-b are represented by A, B, C, D, E, F, G, and H, as shown in FIG. 5.
FIG. 5 illustrates a state wherein detection light beams 11-a and 11-b are incident on the photodetectors 10-a and 10-b. Referring to FIG. 5, regions 12-a and 12-b illustrated on each of the two detection light beams 11-a and 11-b correspond to portions where some light components of a light spot formed on the information recording medium 5 are diffracted by a guide groove or a pit array, which constitutes an information track on the information recording medium 5, and has a phase structure, and .+-.1st-order light components are superposed on a 0th-order detection light beam. As is well known, when a light spot formed on the recording medium 5 falls Just on the information track or the middle point between each two adjacent information tracks, the light amounts on the regions 12-a and 12-b are equal to each other; when the light spot falls outside the information track or the middle point between each two adjacent information tracks, the light amounts of the regions 12-a and 12-b are different from each other. The unbalance between the light amounts of the regions 12-a and 12-b occurs in a direction intersecting the track. Therefore, an AT control signal is obtained by comparing the two light amounts and on the basis of the polarities of a light spot movement and a change in light amount, so that the light spot can correctly follow the information track. Such an AT control signal detection method is executed based on the push-pull method. Note that FIG. 5 illustrates the diffraction distribution regions 12-a and 12-b on the detection light beams, which regions are illustrated on the two detection light beams 11-a and 11-b, correspond to the guide groove of the recording medium, and have different brightness levels, for the sake of simplicity. An arrow 22 in FIG. 5 indicates the direction intersecting the tracks. Each of the two detection light beams 11-a and 11-b has an elliptic shape having the major axis in a direction parallel to the arrow 22. This is because the detection light beam is converged by the cylindrical lens 26 in only a direction parallel to the track, and remains collimated in the track intersection direction (arrow 22). If signal components from these regions are represented by IA, IB, IC, ID, IE, IF, IG, and IH, an AT error signal is obtained by arithmetic processing given by the following equation via differential amplifiers 15-a and 15-b and a summing amplifier 14: EQU AT error signal=(IE-IF)+(IG-IH)
In this example, an AF error signal is detected based on the following principle. More specifically, as in the above-mentioned prior art, the two detection light beams 11-a and 11-b are focused at positions shifted by the same amount W from the plane 25, where the caustic curve of the cylindrical lens 26 is formed, respectively in a direction to approach the convergent lens and in a direction to be separated away from the convergent lens. Therefore, on the photodetectors 10-a and 10-b, the beam spot sizes in the direction parallel to the track change in opposite directions in accordance with a change in focusing state of a light spot for recording/reproduction on the information recording medium. This is because the detection light beam is converged in only the direction parallel to the track, and remains collimated in the track intersection direction (arrow 22). More specifically, when the objective lens is defocused with respect to the recording medium in a certain direction, if the beam spot size increases in the direction parallel to the track on the photodetector 10-a, the beam spot size decreases in the direction parallel to the track on the photodetector 10-b. In an in-focus state, the sizes of the two detection light beams are almost equal to each other, most of the light components are incident on inner light-receiving portions E, F, G, and H, and almost no light components are incident on the outer light-receiving portions A, B, C, and D. When the objective lens is defocused, as described above, many light components are incident on the light-receiving portions A and B and no light components are incident on the light-receiving portions C and D in correspondence with the defocus direction, or vice versa. Therefore, an AF error signal is obtained by the following calculation via summing amplifiers 14-a and 14-b and a differential amplifier 15: EQU AF error signal=(IA+IB)-(IC+ID)
When the AT error signal is obtained by the push-pull method, a position shift between the photodetector and the detection light beam in the track intersection direction poses a problem. In this prior art, a problem posed when a position shift of, e.g., optical parts occurs is explained in an example using a normal convex lens. As shown in FIG. 6(a), the detection light beam is converged by the normal convex lens in a circular pattern, light beam components in the regions 12-a and 12-b are respectively received by the light-receiving portions E and F, and a change in light amount balance is detected by comparing the outputs from these light-receiving portions. When the detection light beam causes the above-mentioned position shift, for example, a light beam component in the region 12-b enters the light-receiving portion for receiving the light beam components in the region 12-a, and light beams in the region 12-a fall outside the corresponding light-receiving portion, as shown in FIG. 6(b). As a result, in the AT error signal, a change in balance occurs in the output signals from the detection portions for receiving light beam components from the regions 12-a and 12-b independently of an actual tracking error, and a DC offset component is generated in the AT error signal. When this DC offset component (to be referred to as an offset hereinafter) is generated, a target value of information tracking control is shifted, and the information track can no longer be correctly tracked.
In the above-mentioned prior art, as a countermeasure against this problem, the detection light beam is not converged in the track intersection direction using the cylindrical lens, as shown in FIG. 5. Therefore, in a section in the track intersection direction, since the detection light beam emerging from the objective lens reaches the light-receiving surface of the photodetector while being left collimated, the beam spot size in the track intersection direction on the light-receiving surface of the photodetector becomes larger than the case using the normal convex lens (see FIG. 6). In this case, since the absolute distance of an optical axis shift on the photodetector, which shift is caused by, e.g., a position shift of optical parts does not change much regardless of the types of lenses, when the cylindrical lens is used, an AT error offset due to the influence of the optical axis shift can be relatively reduced.
In the above-mentioned prior art shown in FIG. 2, a convergent light beam incident on the single photodetector 10 is effective for detecting the AT error signal or detecting both the AF error signal and the AT error signal (i.e., MO signal detection). FIGS. 7 and 8 (FIG. 9 for a case including an MO signal) illustrate patterns of the photodetection regions of the photodetector 10. In FIGS. 7 and 8, the detection light beams are denoted by reference numerals. 11-a and 11-b. In FIG. 7, each of the photodetection regions of the photodetector is divided into two light-receiving portions I and J or K and L, and if signal components from these light-receiving portions are represented by II, IJ, IK, and IL, an AT error signal is obtained by the following arithmetic processing: EQU ATE=(II-IJ)+(IK-IL)
Similarly, in FIG. 8, each of the photodetection regions of the photodetector is divided into six light-receiving portions J, K, L, M, N, and O or T, U, V, X, Y, and Z, and if signal components from these light-receiving portions are represented by IJ, IK, IL, IM, IN, IO, IT, IU, IV, IX, IY, and IZ, an AF error signal is obtained by the following arithmetic processing: EQU AFE=((IK+IN)-(IJ+IL+IM+IO))+((IU+IY)-(IT+IV+IX+IZ))
Also, an AT error signal is obtained by the following arithmetic processing : EQU ATE=((IJ+IK+IL)-(IM+IN+IO))+((IT+IU+IV)-(IX+IY+IZ))
(Similarly, an MO signal is obtained by the following arithmetic processing ) EQU MO=(IJ+IK+IL+IM+IN+IO)-(IT+IU+IV+IX+IY+IZ)
Thus, the above-mentioned prior art respectively suffer from the following problems. Problems in the prior art shown in FIG. 1 are as follows:
(1) The error signal detection is susceptible to the influence of a position shift of the photodetectors due to the structure of the apparatus. More specifically, since the two photodetectors are separately and independently arranged, when the photodetectors cause a position shift due to a change in temperature or a mechanical disturbance, their output signals are unbalanced, and an AT error signal may be undesirably mixed in an AF error signal. Therefore, since an arrangement for preventing a position shift with high reliability must be adopted, the structure is complicated, resulting in an increase in cost.
(2) Since the detection light beam emerging from the objective lens as a collimated beam must be split as it is, the beam splitter must have a size equal to or larger than the diameter of the light beam, and an arrangement around the AF signal detection optical system (even in MO signal detection together with AF signal detection) of an optical head becomes bulky, thus disturbing a compact apparatus. In particular, when an MO signal is differentially detected, the plane of polarization of the detection light beam is rotated through 45.degree. using a halfwave plate. In this case, in order to reduce parts cost by omitting the halfwave plate, the polarization beam splitter and the reflection mirror must be arranged while being rotated through 45.degree., and the optical path of the detection optical system cannot be arranged in a plane, thus also disturbing a compact apparatus.
(3) Since the two photodetectors must be separately arranged at different positions, an arrangement around the AF signal detection optical system (even in MO signal detection together with AF signal detection) of the optical head becomes bulky, thus disturbing a compact apparatus, as described above. In addition, since these photodetectors must be independently adjusted, adjustment cost increases. Also, the structure of a stationary portion of each photodetector is complicated, resulting in high working cost.
(4) Since two each of the photodetectors and convergent lenses must be prepared, parts cost of the apparatus increases.
(5) Each photodetector normally adopts a photodiode. In this case, when the two photodetectors are constituted by totally different parts, since chips must be manufactured after they are cut from wafers, differences between the characteristics of the two photodiodes become large, and this adversely influences arithmetic processing of signals.
(6) If priority is placed on easy AF control in association with the S-curve pattern of an AF error signal, the size of the detection region of each of the two photodetectors is preferably set to be smaller than the beam spot size in an in-focus state. However, when the detection region is set to be smaller than the beam spot size in an in-focus state, the amount of light falling outside the detection region must be abandoned. For this reason, all light components as an information signal returned from the recording medium cannot be detected. This is not preferable for assuring a high MO signal to noise ratio when MO signal detection is also executed.
The prior art shown in FIG. 2 can solve some of problems of the above-mentioned prior art. However, other problems are also posed, and remain unsolved. More specifically, these problems are as follows:
(7) Since the two photodetection regions of the photodetector correspond to and are arranged at the positions shifted from the focal plane of the convergent lens by the same amount in the same direction along the optical axis, a pseudo point (e.g., pseudo zero level) indicating the same signal level as that indicating an in-focus state consequently appears in the obtained AF error signal at a position relatively near an in-focus point in addition to a position truly indicating the in-focus point (see FIG. 10). For this reason, a control system must be designed to discriminate such a pseudo point. In addition, since the direction of a diffracted pattern from a guide groove, which pattern is superposed on the detection light beam on the photodetection region is the same as the information track intersection direction, when the photodetector causes a position shift in the information track intersection direction (the direction of the arrow 22, as shown in FIG. 7), if the light beam 11-a is shifted toward the region 12-a on the light-receiving surface of the photodetector, the light beam 11-b is shifted toward the region 12-b. In this manner, even when an AT error signal obtained from the photodetector 10-a is added to an AT error signal obtained from the photodetector 10-b, the AT error offsets of the two signals are added to each other, and their influences remain (see FIG. 11). The arrow 22 indicates a direction (radial direction) perpendicular to the track of the information recording medium.
(8) Since the detection light beam emerging from the objective lens as a collimated beam must be split as it is, the beam splitter must have a size equal to or larger than the diameter of the light beam, and a compact arrangement around the AF signal detection optical system of an optical head cannot so effectively be realized.
(9) Since the two light beams pass the positions largely separated from the optical axis of the convergent lens, they are easily influenced by aberrations.
(10) The number of types of signal components to be calculated is large, a large number of summing amplifiers and differential amplifiers are required, and the number of times of calculation is increased. As a result, noise increases, and cost of electrical parts rises since a countermeasure against noise must be taken.
In particular, item (7) will be described in detail below with reference to the accompanying drawings. FIGS. 11(a) through 11(c) show AF error signal S-curves each obtained by calculating output signals from the three (or six) light-receiving portions of one of the pair of photodetection regions each including the three (or six) light-receiving portions shown in FIG. 3 (or FIG. 8 or 9). The AF error signal obtained from the three (or six) light-receiving portions of either of the pair of photodetection regions has the same S-curve shape.
In FIG. 10, the level of the AF error signal is assumed to be zero in an in-focus state, for the sake of simplicity. More specifically, assume that the relationship between the size of each photodetection region of the photodetector and the beam spot size, the gains of the pre-amplifiers, and the like are set to attain IV=IU+IW and IY=IX+IZ (or IK+IN=IJ+IL+IM+IO and IU+IY=IT+IV+IX+IZ in the case of FIG. 8 or 9) in an in-focus state (of course, if the level of the AF error signal is not zero in an in-focus state, a problem of appearance of a pseudo in-focus level is similarly posed).
As shown in FIG. 10, a pseudo zero level indicating AF signal level=0 appears near the true in-focus point in the obtained AF error signal S-curve although an in-focus state is not attained. This point is the pseudo in-focus point. Since AF servo control may be erroneously executed at this point, a means for preventing this is required.
The reason why the AF error signal S-curve having the same pattern and including a pseudo zero level can only be obtained from either of the pair of photodetectors, as described above, is that the two photodetectors are arranged at the positions shifted from the focal plane of the convergent lens by the same distance in the same direction along the optical axis. Therefore, even when these two AF error signal S-curves are added to each other, only the amplitude of the S-curve waveform changes, and the pseudo zero level shown in FIG. 10 cannot be eliminated from the AF error signal S-curve.
The prior art shown in FIG. 4 also suffers from some problems. These problems are as follows:
(11) In this prior art, the apparatus is arranged with an eye to lowering sensitivity of the AT error offset with respect to the optical axis shift. More specifically, the beam spot size is not converged in the track intersection direction on the light-receiving surface of the photodetector so as to increase the beam spot size in the track intersection direction, thereby relatively decreasing the optical axis position shift. Therefore, AT offsets caused by optical axis position shifts cannot be perfectly removed. The two detection light beams are projected as beams collimated in the track intersection direction onto the light-receiving surfaces of the photodetectors. Therefore, as has been described above in item (7), when optical axis movement occurs in a certain direction due to an inclination of optical system parts before the light beams reach the photodetectors, for example, when the light beam is shifted toward the region 12-a on the photodetector 10-a in FIG. 5, the light beam is also shifted toward the region 12-a on the photodetector 10-b. As a result, AT error signals obtained from the photodetectors 10-a and 10-b include offsets having the same sign. However, since the two AT error signals are added to each other, and the sum signal is used, the AT error offsets are also added to each other, and the sum offset remains. Consequently, generation of an AT error offset can be suppressed but cannot be removed. In order to detect the AT error signal, the two photodetectors and four photodetection regions are used. However, this arrangement is merely adopted due to a limitation imposed by the arrangement of the AF error signal detection system and has no special effect in the detection of the AT error signal. Rather, this arrangement is disadvantageous in terms of noise caused by an increase in the number of amplifiers in the AT error detection system.
(12) In this prior art as well, when the photodetectors cause a position shift due to a change in temperature or a mechanical disturbance, independent position shifts occur since there are two different photodetectors. More specifically, even when one photodetector causes a position shift, the AF error signal is unbalanced, and the AT error signal may be mixed in the AF error signal. Furthermore, an AT error offset may be generated, and the amplitude of the AT error signal may decrease. In this prior art, although the sensitivity of the AT error offset with respect to the optical axis shift is lowered, another offset generation factor is added due to the arrangement of the two independent photodetectors.
(13) Since the beam spot size corresponds to that of a convergent light beam in a section in a direction parallel to the track on the light-receiving surface of the photodetector, the sensitivity to the optical axis shift in this direction is not low. More specifically, when an optical axis position shift occurs in the direction parallel to the track, since the light beam is converged in the direction parallel to the track on the light-receiving surface of the photodetector, many light beam components fall outside the photodetection region even by a small position shift distance which is allowable in the track intersection direction, thus causing an unbalance of the AF error signal, a decrease in amplitude of the AT error signal, and the like. Even when the sensitivity of generation of an AT offset is lowered with respect to only an optical axis position shift in the track intersection direction, since the sensitivity in the direction parallel to the track is the same as that in the conventional apparatus, it is practically difficult to relax a countermeasure against the optical axis shift.
(14) As is pointed out in the above-mentioned prior art, the detection light beam emerging from the objective lens is converged in the section in only one direction in this prior art as well, and the detection light beam must be split as a collimated beam in the section in the other direction. For this reason, the beam splitter must have a size equal to or larger than the diameter of the collimated beam in at least one direction, and an arrangement around the AF and AT signal detection optical systems of an optical head becomes bulky, thus disturbing a compact apparatus.
The problems of the above prior art (see items (7) and (11)) will be described below with reference to FIGS. 11(a) through 11(c). FIGS. 11(a) and 11(b) schematically show AT signal waveforms obtained from the two split photodetection regions when the light spot intersects a track on the information recording medium 5. Referring to FIGS. 11(a) and (b), offsets .DELTA.1 and .DELTA.2 appear in differential signals of the two-split photodetector when the detection light beams cause position shifts in the same direction with respect to the track intersection direction. In the above-mentioned prior art, differential signals from the photodetectors are added to each other to obtain a final AT error signal. The addition result signal is shown in FIG. 11(c). As indicated by .DELTA.1+.DELTA.2 in FIG. 11(c), offsets of the AT error signals are added to each other, and the sum offset remains, thus adversely influencing tracking precision. In general, when an offset is generated by an optical axis shift, the amplitude of the AT error signal decreases. When an optical axis shift occurs, since AT error signal components cancel each other due to the influence of a neighboring diffraction region of the detection light beam on the two-split photodetector (for example, a light beam in the region 12-b is mixed upon detection of the region 12-a), the amplitude of the AT error signal always decreases, and the influence of the offset is further emphasized relatively. It is, therefore, important to suppress generation of the offset.